Tempstar furnace wont light

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Old 12-22-07, 04:14 PM
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Tempstar furnace wont light

I have a tempstar gas furnace that was made in 1990. When the t-stat kicks on you hear the igniter spark come on but the pilot only stays lit for about 1 second of so. 10 seconds go by and the same thing repeats. The gas at the main burners never starts. this has been an intermittent problem the last few years but now its seem permanent. The unit was working fine this winter until just the other day. The ignition electronic unit is a Honeywell s8600m. Do these things typically malfunction like this or could it be the gas regulator? I am cold and need help!
 
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Old 12-22-07, 04:35 PM
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The pilot flame is having a hard time being proven back at the control board, most likely due to seperate pilot flame sensor next to it or if you do not have a seperate sensor the pilot assembly itself is the sensor, and needs polishing up to get rid of a tarnish. You have to remove the entire pilot assembly to do this. While it is out, you want to also clean the pilot orifice at that end also. Very tiny hole that can cause weak pilot flame and hence no signal back to control board. Cost to you: $0
 
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Old 12-22-07, 05:37 PM
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what do you suggest to use to polish and clean the orafice?
 
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Old 12-23-07, 12:38 AM
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That Honeywell ignition module tends to run like a champion. I installed on in my furnace in 1985, and it's still going strong, even though it was drowned in a flood at one time and dried out in a slow oven!


Still, it could be a bad module.


When the thermostat switches the furnace on, there is an inducer motor in your furnace (if I'm thinking of the right one that should turn on and stay on --- a small fan motor.

A pressure switch verifies that the inducer motor and venting system is operating properly, and turns on power to the ignition module.

The ignition module should turn on the gas for the pilot light and the spark. When the pilot lights, the lit pilot is detected by a process of flame rectification, and the ignition module then turns on the main burner.

The fact that the pilot only stays lit for a second or so suggests that the pressure switch is opening, which shuts off power to the ignition module until the pressure switch closes again.


I'd take an AC voltmeter and connect it from ground to the pressure switch to verify what is happening. One side of the pressure switch should have 24 VAC on it whenever the thermostat is calling for heat. The other side should have 24 VAC switched on when the inducer motor turns on, and it should stay on as long as the thermostat is calling for heat.

If it's shutting off, you have a plugged or damaged heat exchanger, vent system or a plugged or defective pressure switch or associated parts.
 
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Old 12-23-07, 08:56 AM
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thanks seatle pioneer. i will check that out...if i can find my VOM :/
 
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Old 12-26-07, 03:43 PM
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Voltage is good on both sides so i don't thinks its the pressure switch. any other ideas??
 
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Old 12-26-07, 04:06 PM
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Originally Posted by jmixer View Post
what do you suggest to use to polish and clean the orafice?
You use steel wool or a scotchbrite pad on the flame sensor. People say not to use sandpaper but personally I have never had a problem having to go back after I have used fine emory cloth on one. And I have done lots of them.

For the orifice - an orifice is a tiny hole at the flame end of the pilot line. Very tiny. You can try to use air compressor to blow out, or I have used a strand plucked for a wire brush to reem out the hole. Do not use drill bit or anything to enlarge the hole. These holes are smaller than needles or pins.
 
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Old 12-26-07, 04:55 PM
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Originally Posted by jmixer View Post
Voltage is good on both sides so i don't thinks its the pressure switch. any other ideas??

An intermittent poor ground or poor electrical connection could be a problem.


The real solution is to connect an DC microammmeter between the ground connection and the control module and measure the current passing through the pilot flame to operate the flame rectification circuit of the control module.

Gnerally, a good system produces a measurement of 5-7 microamps. A reading of around 1 microamp would often cause the main burner to shut off irregularly. A reading of much less than 1 microamp would typically prevent the burner from turning on at all.

You need to measure that current and then analyze what is causing a marginal current flow if it's around one microamp.
 
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